linux-kernel - Re: [PATCH 4/7] sempahore: add a helper for a concurrency limiter

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Message-ID: <20230331034209.GA12892@google.com>
Date:   Fri, 31 Mar 2023 12:42:09 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>, david@...hat.com,
        patches@...ts.linux.dev, linux-modules@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        petr.pavlu@...e.com, prarit@...hat.com, gregkh@...uxfoundation.org,
        rafael@...nel.org, christophe.leroy@...roup.eu, tglx@...utronix.de,
        song@...nel.org, rppt@...nel.org, willy@...radead.org,
        vbabka@...e.cz, mhocko@...e.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH 4/7] sempahore: add a helper for a concurrency limiter

On (23/03/30 09:23), Linus Torvalds wrote:
> Although we also do have some other issues - I think down_trylock() is
> ok in irq contexts, but mutex_trylock() is not. Maybe that's why
> printk uses semaphores? I forget.

Yes, correct. IIRC we also cannot safely call mutex_unlock() from IRQ
context because it takes some internal mutex spin_lock in a non-IRQ-safe
manner. Semaphore is OK in this regard, both semaphore try_lock() and
unlock() can be called from IRQ.