linux-kernel - Re: write_lock_irq(&tasklist

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Message-ID: <CA+55aFyGHhPvbqGaA0+tx7fbH+Uc6sbGKk2PLoFYCt-agCOgOw@mail.gmail.com>
Date:   Tue, 22 May 2018 13:27:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     psodagud@...eaurora.org
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>, sherryy@...roid.com,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Christoph Lameter <cl@...ux.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sasha Levin <alexander.levin@...izon.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: write_lock_irq(&tasklist_lock)

On Tue, May 22, 2018 at 12:40 PM Sodagudi Prasad <psodagud@...eaurora.org>
wrote:

> Have you observed this type of issues with tasklist_lock ?

tasklist_lock remains pretty painful. It covers too much, but trying to
split it up has never worked well.

It's usually not a huge problem because there are so few writers, but what
you're seeing is the writer starvation issue because readers can be
plentiful.

> Do we need write_lock_irq(&tasklist_lock) in below portion of code ? Can
> I use write_unlock instead of write_lock_irq in portion of code?

You absolutely need write_lock_irq(), because taking the tasklist_lock
without disabling interrupts will deadlock very quickly due to an interrupt
taking the tasklist_lock for reading.

That said, the write_lock_irq(&tasklist_lock) could possibly be replaced
with something like

   static void tasklist_write_lock(void)
   {
         unsigned long flags;
         local_irq_save(flags);
         while (!write_trylock(&tasklist_lock)) {
                 local_irq_restore(flags);
                 do { cpu_relax(); } while (write_islocked(&tasklist_lock));
                 local_irq_disable();
         }
   }

but we don't have that "write_islocked()" function.

So the above would need more work, and is entirely untested anyway,
obviously.

                 Linus