Message-ID: <a23a3226-95d9-9835-c1c7-2d13f4a1ee16@redhat.com>
Date:   Mon, 10 Jan 2022 11:18:27 -0500
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.3.0
Subject: Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems
Content-Language: en-US
To:     Christoph Hellwig <hch@infradead.org>,
        Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        Tim Murray <timmurray@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>
References: <20220108164617.3130175-1-jaegeuk@kernel.org>
 <YdvoxkAAquI17UbX@infradead.org>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YdvoxkAAquI17UbX@infradead.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 1/10/22 03:05, Christoph Hellwig wrote:
> Adding the locking primitive maintainers to this patch adding open coded
> locking primitives..
>
> On Sat, Jan 08, 2022 at 08:46:17AM -0800, Jaegeuk Kim wrote:
>> From: Tim Murray <timmurray@google.com>
>>
>> f2fs rw_semaphores work better if writers can starve readers,
>> especially for the checkpoint thread, because writers are strictly
>> more important than reader threads. This prevents significant priority
>> inversion between low-priority readers that blocked while trying to
>> acquire the read lock and a second acquisition of the write lock that
>> might be blocking high priority work.
>>
>> Signed-off-by: Tim Murray <timmurray@google.com>
>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

We could certainly implement a down_read() variant (e.g. 
down_read_lowprio()) with its own slowpath function to do this within 
the rwsem code as long as there is a good use-case for this kind of 
functionality.

Cheers,
Longman