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Date:   Thu, 20 Dec 2018 13:57:30 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Vineet Gupta <vineet.gupta1@synopsys.com>
Cc:     linux-snps-arc@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/2] ARC: show_regs: avoid page allocator
Message-ID: <20181220125730.GA17350@dhcp22.suse.cz>
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On Tue 18-12-18 10:53:58, Vineet Gupta wrote:
> Use on-stack smaller buffers instead of dynamic pages.
> 
> The motivation for this change was to address lockdep splat when
> signal handling code calls show_regs (with preemption disabled) and
> ARC show_regs calls into sleepable page allocator.
> 
> | potentially unexpected fatal signal 11.
> | BUG: sleeping function called from invalid context at ../mm/page_alloc.c:4317
> | in_atomic(): 1, irqs_disabled(): 0, pid: 57, name: segv
> | no locks held by segv/57.
> | Preemption disabled at:
> | [<8182f17e>] get_signal+0x4a6/0x7c4
> | CPU: 0 PID: 57 Comm: segv Not tainted 4.17.0+ #23
> |
> | Stack Trace:
> |  arc_unwind_core.constprop.1+0xd0/0xf4
> |  __might_sleep+0x1f6/0x234
> |  __get_free_pages+0x174/0xca0
> |  show_regs+0x22/0x330
> |  get_signal+0x4ac/0x7c4     # print_fatal_signals() -> preempt_disable()
> |  do_signal+0x30/0x224
> |  resume_user_mode_begin+0x90/0xd8
> 
> Despite this, lockdep still barfs (see next change), but this patch
> still has merit as in we use smaller/localized buffers now and there's
> less instructoh trace to sift thru when debugging pesky issues.

But show_regs is called from contexts which might be called from deep
call chains (e.g WARN). Is it safe to allocate such a large stack there?
-- 
Michal Hocko
SUSE Labs