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Subject: Re: [PATCH] powerpc/irq: don't use current_stack_pointer() in
 do_IRQ()
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <1bb34d3ea006c308221706290613e6cc5dc3cb74.1575802064.git.christophe.leroy@c-s.fr>
 <20191212125116.GA3381@infradead.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <185df2a7-e6c4-0d2d-59cd-760df94fa3c6@c-s.fr>
Date:   Thu, 12 Dec 2019 17:29:06 +0100
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Le 12/12/2019 à 13:51, Christoph Hellwig a écrit :
> Why can't current_stack_pointer be turned into an inline function using
> inline assembly?  That would reduce the overhead for all callers.
> 

In the old days, it was a macro, and it was changed into an assembly 
function by commit 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bfe9a2cfe91a

It was later renamed from __get_SP() to current_stack_pointer() by 
commit 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=acf620ecf56cfc4edaffaf158250e128539cdd26

But in fact this function is badly named as it doesn't provide the 
current stack pointer but a pointer to the parent's stack frame.

Having it as an extern function forces GCC to set a stack frame in the 
calling function. If inline assembly is used instead, there's a risk of 
not getting a stack frame in the calling function, in which case the 
current_stack_pointer() will return the grandparent's stackframe pointer 
instead of the parent's one.

Christophe