From patchwork Mon Sep 29 15:39:18 2014 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Bernard Ogden X-Patchwork-Id: 3021 Received: (qmail 27034 invoked by alias); 29 Sep 2014 15:39:49 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 26964 invoked by uid 89); 29 Sep 2014 15:39:48 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.6 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_LOW, SPF_PASS autolearn=ham version=3.3.2 X-HELO: mail-wi0-f172.google.com X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references; bh=nb+jIveE5l9SwmPt4KKFZOyTBDyEyhnoqPZ5e8wIm9Q=; b=a22SIbgCtNc1zasfsyB4HlCgjb2z+0NS3ucNQpEMgr8hSWMynrhfsNZMr8kEx2vbur HPtrTv7gEeNHpUnvsSgoG38CAqVbzrgZWdmsHYyh9JQr/WvyWQp8XDnHVyl3KhrX4yF5 1przy4tSDz7Obonanuiz40miKgVKB97fRdtD45wy6AAyOOVcGnuF5L+LC/3DZZieaN+M GWK4Av5aHTz+7aUphrLYUGwPKg/gaZWm4gP4q4HIOXkevPxtIr8e4uMz9yALcacJ+k3m u2dAl3dFBYJAdDc2708su8FjTGBkac23S876cw6k254mg8rLjJG7P+C9dv4YtrXuFSr0 9c5A== X-Gm-Message-State: ALoCoQld4KkskFEgUyGCNNSBwdXdx4bt+Aj0kNcdisrMMsjlStZpSqBWyPh7auzNehg4IvEp7wy/ X-Received: by 10.194.81.101 with SMTP id z5mr45459479wjx.17.1412005180798; Mon, 29 Sep 2014 08:39:40 -0700 (PDT) From: Bernard Ogden To: carlos@redhat.com Cc: libc-alpha@sourceware.org, Bernard Ogden Subject: [PATCH v2] lowlevellock comments Date: Mon, 29 Sep 2014 16:39:18 +0100 Message-Id: <1412005158-24337-1-git-send-email-bernie.ogden@linaro.org> In-Reply-To: <54257E64.1030006@redhat.com> References: <54257E64.1030006@redhat.com> Hi Carlos, Thanks for the review. I think I've addressed all the comments - more specific responses inline below, v2 of patch even further below. Regards, Bernie On 26 September 2014 15:55, Carlos O'Donell wrote: > On 09/26/2014 07:55 AM, Bernard Ogden wrote: > Shouldn't this be "until thread terminates or the wait times out."? Yep, fixed. > Please verify the kernel does not use the private futex operations for this, > and when complete come back and update the comment to say: > > "As of kernel X.Y.Z, the private futex operation is not used." IIUC, the comment is still correct - the relevant code being this snippet (kernel/fork.c:817 in linux-stable): if (tsk->clear_child_tid) { if (!(tsk->flags & PF_SIGNALED) && atomic_read(&mm->mm_users) > 1) { /* * We don't check the error code - if userspace has * not set up a proper pointer then tough luck. */ put_user(0, tsk->clear_child_tid); sys_futex(tsk->clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0); } tsk->clear_child_tid = NULL; } (FUTEX_WAKE without an explicit private flag meaning shared.) I've made the comments for lll_timedwait_tid (lowlevellock.c) and lll_wait_tid (lowlevellock.h) more consistent and hopefully therefore clearer. Is your form of words the idiom for this? - to me it suggests that the private futex operation went out of use in that kernel version, so I changed it a little. >> return oldval; >> >> + /* Put lock into contended state. */ > > Suggest: "Attempt to put lock into contended state." Changed to "Attempt to put lock into state locked with waiters" - on the principle of using consistent terminology everywhere. >> + /* Put lock into contended state. */ > > Suggest: "Attempt to put lock into contended state." Changed to "Attempt to put lock into state locked with waiters" - on the principle of using consistent terminology everywhere. > unneccessary => unnecessary, spell check please. >> + the uncontended cases and hence do not involve syscalls. Unlikely >> + (contended case) paths usually will involve syscalls to wait or to wake. >> + >> + Much of this code takes a 'private' parameter. This may be: >> + LLL_PRIVATE: lock only shared within a process >> + LLL_SHARED: lock may be shared across processes. */ >> + >> +/* The states of locks are: >> + 0 - untaken >> + 1 - taken (by one user) >> + >1 - contended (taken by 1 user, there may be other users waiting) > > I don't think this is true, I think 2 is explicitly "locked, one or more waiters" > > Suggest: > > 0 - unlocked. > 1 - locked, no waiters. > 2 - locked, one or more waiters. I did consider putting '2', but abandoned it because the checks are all for >1 and I haven't proved that nothing in glibc is fiddling the values of futexes to >2. I think we can be in state >1 with no waiters - for example, when the final waiter is woken. To make the >1 state easier to refer to in comments I've gone for: >1 - locked-with-waiters, there _may_ be waiters If I'm right then strictly >1 means locked-possibly-with-waiters, but that's really getting too much of a mouthful (as 'potentially contended' would have been in the earlier terminology). >> + >> + cond_locks are never in the taken state, they are either untaken or >> + contended. > > Please use locked, or unlocked terms everywhere. I've gone for unlocked, locked and locked with waiters. > >> +/* If lock is 0 (untaken), set to 1 (taken). Return 0. Lock acquired. >> + Otherwise lock is unchanged. Return nonzero. Lock not acquired. */ > > Use GNU style variable name and value usage i.e. `lock` is the variable name, > while `LOCK` is the value of `lock`. So you can say "If the value of lock is..." > or "If LOCK is...". > > Suggest: > > If LOCK is 0 (unlocked), set to 1 (locked) and return 0 to indicate the > lock was acquired, otherwise the lock is unchanged and return non-zero > to indicate the lock was not acquired. Split into two sentences, but basically used your suggestion. >> #define lll_trylock(lock) \ >> atomic_compare_and_exchange_bool_acq (&(lock), 1, 0) >> >> +/* If lock is 0 (untaken), set to 2 (contended). Return 0. Lock acquired. >> + Otherwise lock is unchanged. Return nonzero. Lock not acquired. */ > > Please rewrite into full sentences. Done. >> +/* If futex is 0 (untaken), set to 1 (taken). Lock acquired. >> + Otherwise, block until lock is acquired. After blocking, futex is 2 >> + (contended). */ > > Please rewrite into full sentences. Done. >> #define __lll_lock(futex, private) \ >> ((void) \ >> ({ \ >> @@ -52,6 +87,11 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; >> __lll_lock (&(futex), private) >> >> >> +/* If futex is 0 (untaken), set to id (taken). Return 0. Lock acquired. >> + Otherwise, block until either: >> + 1) Lock is acquired. Return 0. futex is id | FUTEX_WAITERS (contended). >> + 2) Previous owner of futex dies. Return value of futex, which is id of >> + dead owner with FUTEX_OWNER_DIED set. Lock not acquired. */ > > Please rewrite into full sentences. Done. >> #define __lll_robust_lock(futex, id, private) \ >> ({ \ >> int *__futex = (futex); \ >> @@ -69,6 +109,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; >> /* This is an expression rather than a statement even though its value is >> void, so that it can be used in a comma expression or as an expression >> that's cast to void. */ >> +/* Unconditionally set futex to 2 (contended). cond locks only have >> + untaken and contended states, so there is no need to check the value of >> + futex before setting. >> + If previous value of futex was 0 (untaken), lock is acquired. >> + Otherwise, block until lock is acquired. futex will still be 2 (contended) >> + after acquisition. */ > > Please rewrite into full sentences. > Done. >> #define __lll_cond_lock(futex, private) \ >> ((void) \ >> ({ \ >> @@ -79,6 +125,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; >> #define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private) >> >> >> +/* If futex is 0 (untaken), set to id | FUTEX_WAITERS (contended). Lock >> + acquired. >> + Otherwise, block until either: >> + 1) Lock is acquired. Return 0. futex is id | FUTEX_WAITERS (contended). >> + 2) Previous owner of futex dies. Return the value of futex, which is id of >> + dead owner with FUTEX_OWNER_DIED set. Lock not acquired. */ > > Please rewrite into full sentences. > Done. >> #define lll_robust_cond_lock(futex, id, private) \ >> __lll_robust_lock (&(futex), (id) | FUTEX_WAITERS, private) >> >> @@ -88,8 +140,7 @@ extern int __lll_timedlock_wait (int *futex, const struct timespec *, >> extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, >> int private) attribute_hidden; >> >> -/* Take futex if it is untaken. >> - Otherwise block until either we get the futex or abstime runs out. */ >> +/* As __lll_lock, but with a timeout. */ > > Can you comment on the return value of the futex if it times out? > Done. >> #define __lll_timedlock(futex, abstime, private) \ >> ({ \ >> int *__futex = (futex); \ >> @@ -104,6 +155,7 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, >> __lll_timedlock (&(futex), abstime, private) >> >> >> +/* As __lll_robust_lock, but with a timeout. */ > > Similarly. > Similarly done. >> #define __lll_robust_timedlock(futex, abstime, id, private) \ >> ({ \ >> int *__futex = (futex); \ >> @@ -121,6 +173,9 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, >> /* This is an expression rather than a statement even though its value is >> void, so that it can be used in a comma expression or as an expression >> that's cast to void. */ >> +/* Unconditionally set futex to 0 (untaken). Lock is released. >> + If previous value of futex was > 1 (contended), wake any waiters. (Waiter >> + that acquires the lock will set futex to >1 (contended).) */ > > Please rewrite into full sentences. > Done. >> #define __lll_unlock(futex, private) \ >> ((void) \ >> ({ \ >> @@ -132,10 +187,12 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, >> #define lll_unlock(futex, private) \ >> __lll_unlock (&(futex), private) >> >> - >> /* This is an expression rather than a statement even though its value is >> void, so that it can be used in a comma expression or as an expression >> that's cast to void. */ >> +/* Unconditionally set futex to 0 (untaken). Lock is released. >> + If previous value of futex had FUTEX_WAITERS set, wake any waiters. (Waiter >> + that acquires the lock will set FUTEX_WAITERS.) */ > > Please rewrite into full sentences. > Done. 2014-09-26 Bernard Ogden * nptl/lowlevellock.c: Add comments. * nptl/lowlevelrobustlock.c: Likewise. * sysdeps/nptl/lowlevellock.h: Likewise. diff --git a/nptl/lowlevellock.c b/nptl/lowlevellock.c index e198af7..c14b24a 100644 --- a/nptl/lowlevellock.c +++ b/nptl/lowlevellock.c @@ -27,10 +27,10 @@ void __lll_lock_wait_private (int *futex) { if (*futex == 2) - lll_futex_wait (futex, 2, LLL_PRIVATE); + lll_futex_wait (futex, 2, LLL_PRIVATE); /* Wait if *futex == 2. */ while (atomic_exchange_acq (futex, 2) != 0) - lll_futex_wait (futex, 2, LLL_PRIVATE); + lll_futex_wait (futex, 2, LLL_PRIVATE); /* Wait if *futex == 2. */ } @@ -40,10 +40,10 @@ void __lll_lock_wait (int *futex, int private) { if (*futex == 2) - lll_futex_wait (futex, 2, private); + lll_futex_wait (futex, 2, private); /* Wait if *futex == 2. */ while (atomic_exchange_acq (futex, 2) != 0) - lll_futex_wait (futex, 2, private); + lll_futex_wait (futex, 2, private); /* Wait if *futex == 2. */ } @@ -75,7 +75,7 @@ __lll_timedlock_wait (int *futex, const struct timespec *abstime, int private) if (rt.tv_sec < 0) return ETIMEDOUT; - /* Wait. */ + /* If *futex == 2, wait until woken or timeout. */ lll_futex_timed_wait (futex, 2, &rt, private); } @@ -83,6 +83,10 @@ __lll_timedlock_wait (int *futex, const struct timespec *abstime, int private) } +/* The kernel notifies a process which uses CLONE_CHILD_CLEARTID via futex + wakeup when the clone terminates. The memory location contains the + thread ID while the clone is running and is reset to zero + afterwards. */ int __lll_timedwait_tid (int *tidp, const struct timespec *abstime) { @@ -113,8 +117,10 @@ __lll_timedwait_tid (int *tidp, const struct timespec *abstime) if (rt.tv_sec < 0) return ETIMEDOUT; - /* Wait until thread terminates. The kernel so far does not use - the private futex operations for this. */ + /* If *tidp == tid, wait until thread terminates or the wait times out. + The kernel to version 3.16.3 does not use the private futex + operations for futex wakeup when the clone terminates. + */ if (lll_futex_timed_wait (tidp, tid, &rt, LLL_SHARED) == -ETIMEDOUT) return ETIMEDOUT; } diff --git a/nptl/lowlevelrobustlock.c b/nptl/lowlevelrobustlock.c index 3525807..470fe00 100644 --- a/nptl/lowlevelrobustlock.c +++ b/nptl/lowlevelrobustlock.c @@ -36,14 +36,17 @@ __lll_robust_lock_wait (int *futex, int private) do { + /* If owner died, return the present value of the futex. */ if (__glibc_unlikely (oldval & FUTEX_OWNER_DIED)) return oldval; + /* Attempt to put lock into state locked with waiters. */ int newval = oldval | FUTEX_WAITERS; if (oldval != newval && atomic_compare_and_exchange_bool_acq (futex, newval, oldval)) continue; + /* If *futex == 2, wait until woken. */ lll_futex_wait (futex, newval, private); try: @@ -100,15 +103,17 @@ __lll_robust_timedlock_wait (int *futex, const struct timespec *abstime, return ETIMEDOUT; #endif - /* Wait. */ + /* If owner died, return the present value of the futex. */ if (__glibc_unlikely (oldval & FUTEX_OWNER_DIED)) return oldval; + /* Attempt to put lock into state locked with waiters. */ int newval = oldval | FUTEX_WAITERS; if (oldval != newval && atomic_compare_and_exchange_bool_acq (futex, newval, oldval)) continue; + /* If *futex == 2, wait until woken or timeout. */ #if (!defined __ASSUME_FUTEX_CLOCK_REALTIME \ || !defined lll_futex_timed_wait_bitset) lll_futex_timed_wait (futex, newval, &rt, private); diff --git a/sysdeps/nptl/lowlevellock.h b/sysdeps/nptl/lowlevellock.h index 28f4ba3..d8c9846 100644 --- a/sysdeps/nptl/lowlevellock.h +++ b/sysdeps/nptl/lowlevellock.h @@ -22,9 +22,40 @@ #include #include +/* Uses futexes to avoid unnecessary calls into the kernel. Likely paths are + the cases with no waiters and hence do not involve syscalls. Unlikely paths + usually will involve syscalls to wait or to wake. + + Much of this code takes a 'private' parameter. This may be: + LLL_PRIVATE: lock only shared within a process + LLL_SHARED: lock may be shared across processes. */ + +/* The states of locks are: + 0 - unlocked + 1 - locked, no waiters + >1 - locked with waiters, there _may_ be waiters + + cond_locks are never in the locked state, they are either unlocked or + locked with waiters. + + The states of robust locks are: + 0 - unlocked + id - locked (by user identified by id & FUTEX_TID_MASK) + + The following flags may be set in the robust lock value: + FUTEX_WAITERS - locked with waiters (there _may_ be waiters) + FUTEX_OWNER_DIED - owning user has exited without releasing the futex. */ + + +/* If LOCK is 0 (unlocked), set to 1 (locked) and return 0 to indicate the + lock was acquired. Otherwise leave the lock unchanged and return non-zero + to indicate the lock was not acquired. */ #define lll_trylock(lock) \ atomic_compare_and_exchange_bool_acq (&(lock), 1, 0) +/* If LOCK is 0 (unlocked), set to 2 (locked with waiters) and return 0 to + indicate the lock was acquired. Otherwise leave the lock unchanged and + return non-zero to indicate the lock was not acquired. */ #define lll_cond_trylock(lock) \ atomic_compare_and_exchange_bool_acq (&(lock), 2, 0) @@ -35,6 +66,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; /* This is an expression rather than a statement even though its value is void, so that it can be used in a comma expression or as an expression that's cast to void. */ +/* The inner conditional compiles to a call to __lll_lock_wait_private if + private is known at compile time to be LLL_PRIVATE, and to a call to + __lll_lock_wait otherwise. */ +/* If FUTEX is 0 (unlocked), set to 1 (locked) and return. Otherwise block + until we acquire the lock, at which point FUTEX is 2 (locked with waiters), + then return. The lock is aways acquired on return. */ #define __lll_lock(futex, private) \ ((void) \ ({ \ @@ -52,6 +89,13 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; __lll_lock (&(futex), private) +/* If FUTEX is 0 (unlocked), set to ID (locked) and return 0 to indicate the + lock was acquired. Otherwise, block until either: + 1) We acquire the lock, at which point FUTEX will be ID | FUTEX_WAITERS + (locked with waiters). Return 0 to indicate that the lock was acquired. + 2) The previous owner of the lock dies. FUTEX will be + ID | FUTEX_OWNER_DIED. FUTEX_WAITERS may also be set. Return FUTEX to + indicate that the lock is not acquired. */ #define __lll_robust_lock(futex, id, private) \ ({ \ int *__futex = (futex); \ @@ -69,6 +113,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; /* This is an expression rather than a statement even though its value is void, so that it can be used in a comma expression or as an expression that's cast to void. */ +/* Unconditionally set FUTEX to 2 (locked with waiters). cond locks only have + unlocked and locked with waiters states, so there is no need to check FUTEX + before setting. + If FUTEX was 0 (unlocked) then return. Otherwise, block until the lock is + acquired, at which point FUTEX is 2 (locked with waiters). The lock is + always acquired on return. */ #define __lll_cond_lock(futex, private) \ ((void) \ ({ \ @@ -79,6 +129,14 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden; #define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private) +/* If FUTEX is 0 (unlocked), set to ID | FUTEX_WAITERS (locked with waiters) + and return, indicating that the lock is acquired. Otherwise, block until + either: + 1) We acquire the lock, at which point FUTEX will be ID | FUTEX_WAITERS + (locked with waiters), then return. + 2) The previous owner of the lock dies. FUTEX will be + ID | FUTEX_OWNER_DIED. FUTEX_WAITERS may also be set. Return FUTEX to + indicate that the lock is not acquired. */ #define lll_robust_cond_lock(futex, id, private) \ __lll_robust_lock (&(futex), (id) | FUTEX_WAITERS, private) @@ -88,8 +146,8 @@ extern int __lll_timedlock_wait (int *futex, const struct timespec *, extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, int private) attribute_hidden; -/* Take futex if it is untaken. - Otherwise block until either we get the futex or abstime runs out. */ +/* As __lll_lock, but with a timeout. If the timeout occurs then return + ETIMEDOUT. If ABSTIME is invalid, return EINVAL. */ #define __lll_timedlock(futex, abstime, private) \ ({ \ int *__futex = (futex); \ @@ -104,6 +162,8 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, __lll_timedlock (&(futex), abstime, private) +/* As __lll_robust_lock, but with a timeout. If the timeout occurs then return + ETIMEDOUT. If ABSTIME is invalid, return EINVAL. */ #define __lll_robust_timedlock(futex, abstime, id, private) \ ({ \ int *__futex = (futex); \ @@ -121,6 +181,9 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, /* This is an expression rather than a statement even though its value is void, so that it can be used in a comma expression or as an expression that's cast to void. */ +/* Unconditionally set FUTEX to 0 (unlocked), releasing the lock. If FUTEX was + >1 (locked with waiters) then wake any waiters. (The waiter who acquires + the lock will set FUTEX to >1.) */ #define __lll_unlock(futex, private) \ ((void) \ ({ \ @@ -132,10 +195,12 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, #define lll_unlock(futex, private) \ __lll_unlock (&(futex), private) - /* This is an expression rather than a statement even though its value is void, so that it can be used in a comma expression or as an expression that's cast to void. */ +/* Unconditionally set FUTEX to 0 (unlocked), releasing the lock. If FUTEX had + FUTEX_WAITERS set then wake any waiters. (The waiter who acquires the lock + will set FUTEX_WAITERS.) */ #define __lll_robust_unlock(futex, private) \ ((void) \ ({ \ @@ -159,15 +224,11 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, #define LLL_LOCK_INITIALIZER (0) #define LLL_LOCK_INITIALIZER_LOCKED (1) -/* The states of a lock are: - 0 - untaken - 1 - taken by one user - >1 - taken by more users */ - /* The kernel notifies a process which uses CLONE_CHILD_CLEARTID via futex wakeup when the clone terminates. The memory location contains the thread ID while the clone is running and is reset to zero - afterwards. */ + afterwards. The kernel to version 3.16.3 does not use the private futex + operations for futex wakeup when the clone terminates. */ #define lll_wait_tid(tid) \ do { \ __typeof (tid) __tid; \ @@ -178,6 +239,8 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *, extern int __lll_timedwait_tid (int *, const struct timespec *) attribute_hidden; +/* As lll_wait_tid, but with a timeout. If the timeout occurs then return + ETIMEDOUT. If ABSTIME is invalid, return EINVAL. */ #define lll_timedwait_tid(tid, abstime) \ ({ \ int __res = 0; \