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Subject[RFC 4/7] x86: Fix possible caching of current_task
Date
this_cpu_read_stable() is allowed and supposed to cache and return the
same value, specifically for current_task. It actually does not cache
current_task very well, which hinders possible invalid caching when the
task is switched in __switch_to().

Fix the possible caching by avoiding the use of current in
__switch_to()'s dynamic extent.

Signed-off-by: Nadav Amit <namit@vmware.com>
---
arch/x86/include/asm/fpu/internal.h | 7 ++++---
arch/x86/include/asm/resctrl_sched.h | 14 +++++++-------
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 ++--
arch/x86/kernel/process_32.c | 4 ++--
arch/x86/kernel/process_64.c | 4 ++--
5 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 4c95c365058a..b537788600fe 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -588,9 +588,10 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)

/*
* Load PKRU from the FPU context if available. Delay loading of the
- * complete FPU state until the return to userland.
+ * complete FPU state until the return to userland. Avoid reading current during
+ * switch.
*/
-static inline void switch_fpu_finish(struct fpu *new_fpu)
+static inline void switch_fpu_finish(struct task_struct *task, struct fpu *new_fpu)
{
u32 pkru_val = init_pkru_value;
struct pkru_state *pk;
@@ -598,7 +599,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
if (!static_cpu_has(X86_FEATURE_FPU))
return;

- set_thread_flag(TIF_NEED_FPU_LOAD);
+ set_ti_thread_flag(task_thread_info(task), TIF_NEED_FPU_LOAD);

if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
return;
diff --git a/arch/x86/include/asm/resctrl_sched.h b/arch/x86/include/asm/resctrl_sched.h
index f6b7fe2833cc..9a00d9df9d02 100644
--- a/arch/x86/include/asm/resctrl_sched.h
+++ b/arch/x86/include/asm/resctrl_sched.h
@@ -51,7 +51,7 @@ DECLARE_STATIC_KEY_FALSE(rdt_mon_enable_key);
* simple as possible.
* Must be called with preemption disabled.
*/
-static void __resctrl_sched_in(void)
+static void __resctrl_sched_in(struct task_struct *task)
{
struct resctrl_pqr_state *state = this_cpu_ptr(&pqr_state);
u32 closid = state->default_closid;
@@ -62,13 +62,13 @@ static void __resctrl_sched_in(void)
* Else use the closid/rmid assigned to this cpu.
*/
if (static_branch_likely(&rdt_alloc_enable_key)) {
- if (current->closid)
+ if (task->closid)
closid = current->closid;
}

if (static_branch_likely(&rdt_mon_enable_key)) {
- if (current->rmid)
- rmid = current->rmid;
+ if (task->rmid)
+ rmid = task->rmid;
}

if (closid != state->cur_closid || rmid != state->cur_rmid) {
@@ -78,15 +78,15 @@ static void __resctrl_sched_in(void)
}
}

-static inline void resctrl_sched_in(void)
+static inline void resctrl_sched_in(struct task_struct *task)
{
if (static_branch_likely(&rdt_enable_key))
- __resctrl_sched_in();
+ __resctrl_sched_in(task);
}

#else

-static inline void resctrl_sched_in(void) {}
+static inline void resctrl_sched_in(struct task_struct *task) {}

#endif /* CONFIG_X86_CPU_RESCTRL */

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index bf3034994754..71bd82a6e3c6 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -311,7 +311,7 @@ static void update_cpu_closid_rmid(void *info)
* executing task might have its own closid selected. Just reuse
* the context switch code.
*/
- resctrl_sched_in();
+ resctrl_sched_in(current);
}

/*
@@ -536,7 +536,7 @@ static void move_myself(struct callback_head *head)

preempt_disable();
/* update PQR_ASSOC MSR to make resource group go into effect */
- resctrl_sched_in();
+ resctrl_sched_in(current);
preempt_enable();

kfree(callback);
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index b8ceec4974fe..699a4c95ab13 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -292,10 +292,10 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)

this_cpu_write(current_task, next_p);

- switch_fpu_finish(next_fpu);
+ switch_fpu_finish(next_p, next_fpu);

/* Load the Intel cache allocation PQR MSR. */
- resctrl_sched_in();
+ resctrl_sched_in(next_p);

return prev_p;
}
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 250e4c4ac6d9..e945bc744804 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -575,7 +575,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
this_cpu_write(current_task, next_p);
this_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p));

- switch_fpu_finish(next_fpu);
+ switch_fpu_finish(next_p, next_fpu);

/* Reload sp0. */
update_task_stack(next_p);
@@ -622,7 +622,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
}

/* Load the Intel cache allocation PQR MSR. */
- resctrl_sched_in();
+ resctrl_sched_in(next_p);

return prev_p;
}
--
2.17.1
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