user/sven/linux.git/kernel/locking/rwsem-xadd.c, branch v4.4.27

locking/rwsem: Use acquire/release semantics

2015-10-06T15:28:24Z

As of 654672d4ba1 (locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations) and 6d79ef2d30e (locking, asm-generic: Add _{relaxed|acquire|release}() variants for 'atomic_long_t'), weakly ordered archs can benefit from more relaxed use of barriers when locking and unlocking, instead of regular full barrier semantics. While currently only arm64 supports such optimizations, updating corresponding locking primitives serves for other archs to immediately benefit as well, once the necessary machinery is implemented of course. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Thomas Gleixner Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Paul E.McKenney Cc: Peter Zijlstra Cc: Will Deacon Cc: linux-kernel@vger.kernel.org Link: http://lkml.kernel.org/r/1443643395-17016-6-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Reduce spinlock contention in wakeup after up_read()/up_write()

2015-05-08T10:27:59Z

In up_write()/up_read(), rwsem_wake() will be called whenever it detects that some writers/readers are waiting. The rwsem_wake() function will take the wait_lock and call __rwsem_do_wake() to do the real wakeup. For a heavily contended rwsem, doing a spin_lock() on wait_lock will cause further contention on the heavily contended rwsem cacheline resulting in delay in the completion of the up_read/up_write operations. This patch makes the wait_lock taking and the call to __rwsem_do_wake() optional if at least one spinning writer is present. The spinning writer will be able to take the rwsem and call rwsem_wake() later when it calls up_write(). With the presence of a spinning writer, rwsem_wake() will now try to acquire the lock using trylock. If that fails, it will just quit. Suggested-by: Peter Zijlstra (Intel) Signed-off-by: Waiman Long Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Davidlohr Bueso Acked-by: Jason Low Cc: Andrew Morton Cc: Borislav Petkov Cc: Douglas Hatch Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Scott J Norton Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1430428337-16802-2-git-send-email-Waiman.Long@hp.com Signed-off-by: Ingo Molnar

locking/rwsem: Fix lock optimistic spinning when owner is not running

2015-03-07T08:50:49Z

Ming reported soft lockups occurring when running xfstest due to the following tip:locking/core commit: b3fd4f03ca0b ("locking/rwsem: Avoid deceiving lock spinners") When doing optimistic spinning in rwsem, threads should stop spinning when the lock owner is not running. While a thread is spinning on owner, if the owner reschedules, owner->on_cpu returns false and we stop spinning. However, this commit essentially caused the check to get ignored because when we break out of the spin loop due to !on_cpu, we continue spinning if sem->owner != NULL. This patch fixes this by making sure we stop spinning if the owner is not running. Furthermore, just like with mutexes, refactor the code such that we don't have separate checks for owner_running(). This makes it more straightforward in terms of why we exit the spin on owner loop and we would also avoid needing to "guess" why we broke out of the loop to make this more readable. Reported-and-tested-by: Ming Lei Signed-off-by: Jason Low Acked-by: Davidlohr Bueso Cc: Andrew Morton Cc: Dave Jones Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Oleg Nesterov Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Sasha Levin Cc: Thomas Gleixner Cc: Tim Chen Link: http://lkml.kernel.org/r/1425714331.2475.388.camel@j-VirtualBox Signed-off-by: Ingo Molnar

locking: Remove ACCESS_ONCE() usage

2015-02-24T07:44:16Z

With the new standardized functions, we can replace all ACCESS_ONCE() calls across relevant locking - this includes lockref and seqlock while at it. ACCESS_ONCE() does not work reliably on non-scalar types. For example gcc 4.6 and 4.7 might remove the volatile tag for such accesses during the SRA (scalar replacement of aggregates) step: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 Update the new calls regardless of if it is a scalar type, this is cleaner than having three alternatives. Signed-off-by: Davidlohr Bueso Cc: Peter Zijlstra Cc: Linus Torvalds Cc: Andrew Morton Cc: Thomas Gleixner Cc: Paul E. McKenney Link: http://lkml.kernel.org/r/1424662301.6539.18.camel@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Check for active lock before bailing on spinning

2015-02-18T15:57:18Z

37e9562453b ("locking/rwsem: Allow conservative optimistic spinning when readers have lock") forced the default for optimistic spinning to be disabled if the lock owner was nil, which makes much sense for readers. However, while it is not our priority, we can make some optimizations for write-mostly workloads. We can bail the spinning step and still be conservative if there are any active tasks, otherwise there's really no reason not to spin, as the semaphore is most likely unlocked. This patch recovers most of a Unixbench 'execl' benchmark throughput by sleeping less and making better average system usage: before: CPU %user %nice %system %iowait %steal %idle all 0.60 0.00 8.02 0.00 0.00 91.38 after: CPU %user %nice %system %iowait %steal %idle all 1.22 0.00 70.18 0.00 0.00 28.60 Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Acked-by: Jason Low Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Paul E. McKenney Cc: Tim Chen Link: http://lkml.kernel.org/r/1422609267-15102-6-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Avoid deceiving lock spinners

2015-02-18T15:57:16Z

When readers hold the semaphore, the ->owner is nil. As such, and unlike mutexes, '!owner' does not necessarily imply that the lock is free. This will cause writers to potentially spin excessively as they've been mislead to thinking they have a chance of acquiring the lock, instead of blocking. This patch therefore enhances the counter check when the owner is not set by the time we've broken out of the loop. Otherwise we can return true as a new owner has the lock and thus we want to continue spinning. While at it, we can make rwsem_spin_on_owner() less ambiguos and return right away under need_resched conditions. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Jason Low Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Paul E. McKenney Cc: Tim Chen Link: http://lkml.kernel.org/r/1422609267-15102-5-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Set lock ownership ASAP

2015-02-18T15:57:13Z

In order to optimize the spinning step, we need to set the lock owner as soon as the lock is acquired; after a successful counter cmpxchg operation, that is. This is particularly useful as rwsems need to set the owner to nil for readers, so there is a greater chance of falling out of the spinning. Currently we only set the owner much later in the game, in the more generic level -- latency can be specially bad when waiting for a node->next pointer when releasing the osq in up_write calls. As such, update the owner inside rwsem_try_write_lock (when the lock is obtained after blocking) and rwsem_try_write_lock_unqueued (when the lock is obtained while spinning). This requires creating a new internal rwsem.h header to share the owner related calls. Also cleanup some headers for mutex and rwsem. Suggested-by: Peter Zijlstra Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Jason Low Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Paul E. McKenney Cc: Tim Chen Link: http://lkml.kernel.org/r/1422609267-15102-4-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Document barrier need when waking tasks

2015-02-18T15:57:10Z

The need for the smp_mb() in __rwsem_do_wake() should be properly documented. Applies to both xadd and spinlock variants. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Jason Low Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Paul E. McKenney Cc: Tim Chen Link: http://lkml.kernel.org/r/1422609267-15102-3-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Use task->state helpers

2015-02-04T06:57:39Z

Call __set_task_state() instead of assigning the new state directly. These interfaces also aid CONFIG_DEBUG_ATOMIC_SLEEP environments, keeping track of who last changed the state. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: "Paul E. McKenney" Cc: Jason Low Cc: Michel Lespinasse Cc: Tim Chen Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1422257769-14083-2-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar

locking/rwsem: Avoid double checking before try acquiring write lock

2014-10-03T04:09:29Z

Commit 9b0fc9c09f1b ("rwsem: skip initial trylock in rwsem_down_write_failed") checks for if there are known active lockers in order to avoid write trylocking using expensive cmpxchg() when it likely wouldn't get the lock. However, a subsequent patch was added such that we directly check for sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg(). Thus, commit 9b0fc9c09f1b now just adds overhead. This patch modifies it so that we only do a check for if count == RWSEM_WAITING_BIAS. Also, add a comment on why we do an "extra check" of count before the cmpxchg(). Signed-off-by: Jason Low Acked-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Aswin Chandramouleeswaran Cc: Chegu Vinod Cc: Peter Hurley Cc: Tim Chen Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1410913017.2447.22.camel@j-VirtualBox Signed-off-by: Ingo Molnar