From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>,
Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Chris Mason <clm@fb.com>, Darren Hart <dvhart@linux.intel.com>,
linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get_key()
Date: Tue, 5 Jan 2016 12:43:42 -0800 [thread overview]
Message-ID: <20160105204342.GA15465@linux-uzut.site> (raw)
In-Reply-To: <20160105203317.GQ6344@twins.programming.kicks-ass.net>
On Tue, 05 Jan 2016, Peter Zijlstra wrote:
>On Tue, Jan 05, 2016 at 12:23:55PM -0800, Davidlohr Bueso wrote:
>> + if (unlikely(!mapping)) {
>> + int shmem_swizzled;
>> +
>> + /*
>> + * Page lock is required to identify which special case above
>> + * applies. If this is really a shmem page then the page lock
>> + * will prevent unexpected transitions.
>> + */
>> + lock_page(page);
>> + shmem_swizzled = PageSwapCache(page);
>> unlock_page(page);
>> put_page(page);
>> + WARN_ON_ONCE(mapping);
>
>We've not re-loaded mapping, so how could this possibly be?
Yep, this wants to be page->mapping.
>
>
>> + /*
>> + * Take a reference unless it is about to be freed. Previously
>> + * this reference was taken by ihold under the page lock
>> + * pinning the inode in place so i_lock was unnecessary. The
>> + * only way for this check to fail is if the inode was
>> + * truncated in parallel so warn for now if this happens.
>> + *
>> + * TODO: VFS and/or filesystem people should review this check
>> + * and see if there is a safer or more reliable way to do this.
>> + */
>> + if (WARN_ON(!atomic_inc_not_zero(&inode->i_count))) {
>> + rcu_read_unlock();
>> + put_page(page);
>> + goto again;
>> + }
>> +
>> + /*
>> + * get_futex_key() must imply MB (B) and we are not going to
>> + * call into get_futex_key_refs() at this point.
>> + */
>> + smp_mb__after_atomic();
>
>I don't get this one, the above is a successful atomic op with return
>value, that _must_ imply a full barrier.
Ah sure, I was actually following convention of what we have for our plain atomic_inc,
but in this case we are returning a value, so yeah, it is not required. Will drop.
Thanks,
Davidlohr
next prev parent reply other threads:[~2016-01-05 20:43 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-05 20:23 [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get_key() Davidlohr Bueso
2016-01-05 20:33 ` Peter Zijlstra
2016-01-05 20:43 ` Davidlohr Bueso [this message]
2016-01-05 21:11 ` Davidlohr Bueso
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