From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
linux-kernel@vger.kernel.org, x86@kernel.org,
Davidlohr Bueso <dave@stgolabs.net>,
Linus Torvalds <torvalds@linux-foundation.org>,
Tim Chen <tim.c.chen@linux.intel.com>,
huang ying <huang.ying.caritas@gmail.com>,
stable@vger.kernel.org
Subject: Re: [PATCH-tip v7 01/20] locking/rwsem: Prevent decrement of reader count before increment
Date: Fri, 3 May 2019 14:06:56 +0200 [thread overview]
Message-ID: <20190503120656.GD2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190428212557.13482-2-longman@redhat.com>
On Sun, Apr 28, 2019 at 05:25:38PM -0400, Waiman Long wrote:
> During my rwsem testing, it was found that after a down_read(), the
> reader count may occasionally become 0 or even negative. Consequently,
> a writer may steal the lock at that time and execute with the reader
> in parallel thus breaking the mutual exclusion guarantee of the write
> lock. In other words, both readers and writer can become rwsem owners
> simultaneously.
>
> The current reader wakeup code does it in one pass to clear waiter->task
> and put them into wake_q before fully incrementing the reader count.
> Once waiter->task is cleared, the corresponding reader may see it,
> finish the critical section and do unlock to decrement the count before
> the count is incremented. This is not a problem if there is only one
> reader to wake up as the count has been pre-incremented by 1. It is
> a problem if there are more than one readers to be woken up and writer
> can steal the lock.
>
> The wakeup was actually done in 2 passes before the v4.9 commit
> 70800c3c0cc5 ("locking/rwsem: Scan the wait_list for readers only
> once"). To fix this problem, the wakeup is now done in two passes
> again. In the first pass, we collect the readers and count them. The
> reader count is then fully incremented. In the second pass, the
> waiter->task is then cleared and they are put into wake_q to be woken
> up later.
>
> Fixes: 70800c3c0cc5 ("locking/rwsem: Scan the wait_list for readers only once")
It is effectively a revert of that patch, right? Just written more
clever.
next prev parent reply other threads:[~2019-05-03 12:07 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-28 21:25 [PATCH-tip v7 00/20] locking/rwsem: Rwsem rearchitecture part 2 Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 01/20] locking/rwsem: Prevent decrement of reader count before increment Waiman Long
2019-05-03 12:06 ` Peter Zijlstra [this message]
2019-05-03 13:32 ` Waiman Long
2019-05-07 7:07 ` [tip:locking/urgent] " tip-bot for Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 02/20] locking/rwsem: Make owner available even if !CONFIG_RWSEM_SPIN_ON_OWNER Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 03/20] locking/rwsem: Remove rwsem_wake() wakeup optimization Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 04/20] locking/rwsem: Implement a new locking scheme Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 05/20] locking/rwsem: Merge rwsem.h and rwsem-xadd.c into rwsem.c Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 06/20] locking/rwsem: Code cleanup after files merging Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 07/20] locking/rwsem: Make rwsem_spin_on_owner() return owner state Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 08/20] locking/rwsem: Implement lock handoff to prevent lock starvation Waiman Long
2019-05-03 13:10 ` Peter Zijlstra
2019-05-03 13:57 ` Waiman Long
2019-05-03 14:37 ` David Laight
2019-04-28 21:25 ` [PATCH-tip v7 09/20] locking/rwsem: Always release wait_lock before waking up tasks Waiman Long
2019-05-03 13:37 ` Peter Zijlstra
2019-05-03 13:56 ` Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 10/20] locking/rwsem: More optimal RT task handling of null owner Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 11/20] locking/rwsem: Wake up almost all readers in wait queue Waiman Long
2019-05-03 16:51 ` Peter Zijlstra
2019-05-03 17:15 ` Waiman Long
2019-05-06 11:49 ` Peter Zijlstra
2019-04-28 21:25 ` [PATCH-tip v7 12/20] locking/rwsem: Clarify usage of owner's nonspinaable bit Waiman Long
2019-05-03 15:21 ` Peter Zijlstra
2019-05-03 15:26 ` Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 13/20] locking/rwsem: Enable readers spinning on writer Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 14/20] locking/rwsem: Enable time-based spinning on reader-owned rwsem Waiman Long
2019-05-06 15:47 ` Peter Zijlstra
2019-04-28 21:25 ` [PATCH-tip v7 15/20] locking/rwsem: Adaptive disabling of reader optimistic spinning Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 16/20] locking/rwsem: Add more rwsem owner access helpers Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 17/20] locking/rwsem: Guard against making count negative Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 18/20] locking/rwsem: Merge owner into count on x86-64 Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 19/20] locking/rwsem: Remove redundant computation of writer lock word Waiman Long
2019-04-28 21:25 ` [PATCH-tip v7 20/20] locking/rwsem: Disable preemption in down_read*() if owner in count Waiman Long
2019-04-28 22:46 ` [PATCH-tip v7 00/20] locking/rwsem: Rwsem rearchitecture part 2 Linus Torvalds
2019-04-28 23:12 ` Waiman Long
2019-04-28 23:19 ` Waiman Long
2019-04-29 0:10 ` Linus Torvalds
2019-04-29 0:27 ` Waiman Long
2019-04-29 2:41 ` Linus Torvalds
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