From 870568b39064cab2dd971fe57969916036982862 Mon Sep 17 00:00:00 2001 From: Suresh Siddha Date: Mon, 2 Jun 2008 15:57:27 -0700 Subject: [PATCH] x86, fpu: fix CONFIG_PREEMPT=y corruption of application's FPU stack MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Jürgen Mell reported an FPU state corruption bug under CONFIG_PREEMPT, and bisected it to commit v2.6.19-1363-gacc2076, "i386: add sleazy FPU optimization". Add tsk_used_math() checks to prevent calling math_state_restore() which can sleep in the case of !tsk_used_math(). This prevents making a blocking call in __switch_to(). Apparently "fpu_counter > 5" check is not enough, as in some signal handling and fork/exec scenarios, fpu_counter > 5 and !tsk_used_math() is possible. It's a side effect though. This is the failing scenario: process 'A' in save_i387_ia32() just after clear_used_math() Got an interrupt and pre-empted out. At the next context switch to process 'A' again, kernel tries to restore the math state proactively and sees a fpu_counter > 0 and !tsk_used_math() This results in init_fpu() during the __switch_to()'s math_state_restore() And resulting in fpu corruption which will be saved/restored (save_i387_fxsave and restore_i387_fxsave) during the remaining part of the signal handling after the context switch. Bisected-by: Jürgen Mell Signed-off-by: Suresh Siddha Tested-by: Jürgen Mell Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner Cc: stable@kernel.org --- Reading git-format-patch failed