From 870568b39064cab2dd971fe57969916036982862 Mon Sep 17 00:00:00 2001 From: Suresh Siddha <suresh.b.siddha@intel.com> 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 <j.mell@t-online.de> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com> Tested-by: Jürgen Mell <j.mell@t-online.de> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: stable@kernel.org --- arch/x86/kernel/process_32.c | 5 ++++- arch/x86/kernel/process_64.c | 5 ++++- 2 files changed, 8 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c index f8476dfbb60d..6d5483356e74 100644 --- a/arch/x86/kernel/process_32.c +++ b/arch/x86/kernel/process_32.c @@ -649,8 +649,11 @@ struct task_struct * __switch_to(struct task_struct *prev_p, struct task_struct /* If the task has used fpu the last 5 timeslices, just do a full * restore of the math state immediately to avoid the trap; the * chances of needing FPU soon are obviously high now + * + * tsk_used_math() checks prevent calling math_state_restore(), + * which can sleep in the case of !tsk_used_math() */ - if (next_p->fpu_counter > 5) + if (tsk_used_math(next_p) && next_p->fpu_counter > 5) math_state_restore(); /* diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index e2319f39988b..ac54ff56df80 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -658,8 +658,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) /* If the task has used fpu the last 5 timeslices, just do a full * restore of the math state immediately to avoid the trap; the * chances of needing FPU soon are obviously high now + * + * tsk_used_math() checks prevent calling math_state_restore(), + * which can sleep in the case of !tsk_used_math() */ - if (next_p->fpu_counter>5) + if (tsk_used_math(next_p) && next_p->fpu_counter > 5) math_state_restore(); return prev_p; } -- 2.34.1