From: Andreas Dilger <adilger@dilger.ca>
To: Jan Kara <jack@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
Waiman Long <longman@redhat.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Jan Kara <jack@suse.com>, Jeff Layton <jlayton@poochiereds.net>,
"J. Bruce Fields" <bfields@fieldses.org>,
Tejun Heo <tj@kernel.org>,
Christoph Lameter <cl@linux-foundation.org>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Andi Kleen <andi@firstfloor.org>,
Dave Chinner <dchinner@redhat.com>,
Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty()
Date: Tue, 7 Nov 2017 10:59:29 -0700 [thread overview]
Message-ID: <C189B9DE-0845-4EC9-9D96-4815EF9E3F8B@dilger.ca> (raw)
In-Reply-To: <20171107115921.GC11391@quack2.suse.cz>
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On Nov 7, 2017, at 4:59 AM, Jan Kara <jack@suse.cz> wrote:
> On Mon 06-11-17 10:47:08, Davidlohr Bueso wrote:
>> + /*
>> + * Serialize dlist->used_lists such that a 0->1 transition is not
>> + * missed by another thread checking if any of the dlock lists are
>> + * used.
>> + *
>> + * CPU0 CPU1
>> + * dlock_list_add() dlock_lists_empty()
>> + * [S] atomic_inc(used_lists);
>> + * smp_mb__after_atomic();
>> + * smp_mb__before_atomic();
>> + * [L] atomic_read(used_lists)
>> + * list_add()
>> + */
>> + smp_mb__before_atomic();
>> + return !atomic_read(&dlist->used_lists);
Just a general kernel programming question here - I thought the whole point
of atomics is that they are, well, atomic across all CPUs so there is no
need for a memory barrier? If there is a need for a memory barrier for
each atomic access (assuming it isn't accessed under another lock, which would
make the use of atomic types pointless, IMHO) then I'd think there is a lot
of code in the kernel that isn't doing this properly.
What am I missing here?
I don't see how this helps if the operations are executed like:
* CPU0 CPU1
* dlock_list_add() dlock_lists_empty()
* [S] atomic_inc(used_lists);
* smp_mb__before_atomic();
* smp_mb__after_atomic();
* [L] atomic_read(used_lists)
or alternately like:
* CPU0 CPU1
* dlock_list_add() dlock_lists_empty()
* smp_mb__before_atomic();
* [S] atomic_inc(used_lists);
* smp_mb__after_atomic();
* [L] atomic_read(used_lists)
then the same problem would exist, unless those functions/macros are somehow
bound to the atomic operations themselves? In that case, what makes the use
of atomic_{inc,dec,read}() in other parts of the code safe without them?
Cheers, Andreas
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next prev parent reply other threads:[~2017-11-07 17:59 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-31 18:50 [PATCH v8 0/6] vfs: Use dlock list for SB's s_inodes list Waiman Long
2017-10-31 18:50 ` [PATCH v8 1/6] lib/dlock-list: Distributed and lock-protected lists Waiman Long
2017-10-31 21:37 ` Davidlohr Bueso
2017-11-01 18:44 ` Waiman Long
2017-11-02 17:04 ` Davidlohr Bueso
2017-11-02 17:30 ` Waiman Long
2017-11-03 13:34 ` Davidlohr Bueso
2017-11-03 14:22 ` [PATCH v3] lib/dlock-list: Scale dlock_lists_empty() Davidlohr Bueso
2017-11-03 16:33 ` Waiman Long
2017-11-06 18:47 ` [PATCH v4] " Davidlohr Bueso
2017-11-06 19:06 ` Waiman Long
2017-11-07 11:59 ` Jan Kara
2017-11-07 17:59 ` Andreas Dilger [this message]
2017-11-07 18:57 ` Waiman Long
2017-11-07 19:36 ` James Bottomley
2017-11-08 2:08 ` Boqun Feng
2017-11-09 17:24 ` Davidlohr Bueso
2017-11-09 17:30 ` Peter Zijlstra
2017-11-29 15:29 ` [PATCH v8 1/6] lib/dlock-list: Distributed and lock-protected lists Davidlohr Bueso
2017-10-31 18:50 ` [PATCH v8 2/6] vfs: Remove unnecessary list_for_each_entry_safe() variants Waiman Long
2017-10-31 18:50 ` [PATCH v8 3/6] vfs: Use dlock list for superblock's inode list Waiman Long
2017-10-31 18:50 ` [PATCH v8 4/6] lib/dlock-list: Make sibling CPUs share the same linked list Waiman Long
2017-11-01 8:38 ` Jan Kara
2017-10-31 18:50 ` [PATCH v8 5/6] lib/dlock-list: Enable faster lookup with hashing Waiman Long
2017-11-01 8:40 ` Jan Kara
2017-11-01 13:16 ` Waiman Long
2017-10-31 18:51 ` [PATCH v8 6/6] lib/dlock-list: Add an IRQ-safe mode to be used in interrupt handler Waiman Long
2017-10-31 21:29 ` Davidlohr Bueso
2017-11-29 15:26 ` [PATCH v8 0/6] vfs: Use dlock list for SB's s_inodes list Davidlohr Bueso
2017-11-29 15:31 ` Waiman Long
2018-02-26 2:47 ` Dave Chinner
2018-02-26 4:05 ` Waiman Long
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