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SubjectRe: [RFC][PATCH 3/3] sched: User Mode Concurency Groups
On Fri, Jan 14, 2022 at 03:09:55PM +0100, Peter Zijlstra wrote:
>
> Hi!
>
> I've seen you send a new version based on this, but I figured I ought to
> reply to this first.
>
> On Tue, Dec 21, 2021 at 05:19:00PM +0000, Peter Oskolkov wrote:
>
> > > +/* pre-schedule() */
> > > +void umcg_wq_worker_sleeping(struct task_struct *tsk)
> > > +{
> > > + struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
> > > +
> > > + /* Must not fault, mmap_sem might be held. */
> > > + pagefault_disable();
> > > +
> > > + if (WARN_ON_ONCE(!tsk->umcg_server))
> > > + UMCG_DIE_PF("no server");
> >
> > We can get here if a running worker (no pinned pages) gets a pagefault
> > in the userspace. Is umcg_sys_enter() called for pagefaults? If not,
> > we should not kill the worker; also the userspace won't be able to
> > detect this worker blocking on a pagefault...
>
> Ufff.. good one. No #PF doesn't pass through sys_enter, I'll have to go
> fix that.

Something like the below I think.. it builds, but I've not yet tested
it.


---
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -73,18 +73,6 @@

DECLARE_BITMAP(system_vectors, NR_VECTORS);

-static inline void cond_local_irq_enable(struct pt_regs *regs)
-{
- if (regs->flags & X86_EFLAGS_IF)
- local_irq_enable();
-}
-
-static inline void cond_local_irq_disable(struct pt_regs *regs)
-{
- if (regs->flags & X86_EFLAGS_IF)
- local_irq_disable();
-}
-
__always_inline int is_valid_bugaddr(unsigned long addr)
{
if (addr < TASK_SIZE_MAX)
@@ -177,9 +165,9 @@ static void do_error_trap(struct pt_regs

if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
NOTIFY_STOP) {
- cond_local_irq_enable(regs);
+ irqentry_irq_enable(regs);
do_trap(trapnr, signr, str, regs, error_code, sicode, addr);
- cond_local_irq_disable(regs);
+ irqentry_irq_disable(regs);
}
}

@@ -300,7 +288,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_alignment_
if (!user_mode(regs))
die("Split lock detected\n", regs, error_code);

- local_irq_enable();
+ irqentry_irq_enable(regs);

if (handle_user_split_lock(regs, error_code))
goto out;
@@ -309,7 +297,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_alignment_
error_code, BUS_ADRALN, NULL);

out:
- local_irq_disable();
+ irqentry_irq_disable(regs);
}

#ifdef CONFIG_VMAP_STACK
@@ -473,14 +461,14 @@ DEFINE_IDTENTRY(exc_bounds)
if (notify_die(DIE_TRAP, "bounds", regs, 0,
X86_TRAP_BR, SIGSEGV) == NOTIFY_STOP)
return;
- cond_local_irq_enable(regs);
+ irqentry_irq_enable(regs);

if (!user_mode(regs))
die("bounds", regs, 0);

do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, 0, 0, NULL);

- cond_local_irq_disable(regs);
+ irqentry_irq_disable(regs);
}

enum kernel_gp_hint {
@@ -567,7 +555,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_pr
unsigned long gp_addr;
int ret;

- cond_local_irq_enable(regs);
+ irqentry_irq_enable(regs);

if (static_cpu_has(X86_FEATURE_UMIP)) {
if (user_mode(regs) && fixup_umip_exception(regs))
@@ -638,7 +626,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_pr
die_addr(desc, regs, error_code, gp_addr);

exit:
- cond_local_irq_disable(regs);
+ irqentry_irq_disable(regs);
}

static bool do_int3(struct pt_regs *regs)
@@ -665,9 +653,9 @@ static void do_int3_user(struct pt_regs
if (do_int3(regs))
return;

- cond_local_irq_enable(regs);
+ irqentry_irq_enable(regs);
do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, 0, 0, NULL);
- cond_local_irq_disable(regs);
+ irqentry_irq_disable(regs);
}

DEFINE_IDTENTRY_RAW(exc_int3)
@@ -1003,7 +991,7 @@ static __always_inline void exc_debug_us
goto out;

/* It's safe to allow irq's after DR6 has been saved */
- local_irq_enable();
+ irqentry_irq_enable(regs);

if (v8086_mode(regs)) {
handle_vm86_trap((struct kernel_vm86_regs *)regs, 0, X86_TRAP_DB);
@@ -1020,7 +1008,7 @@ static __always_inline void exc_debug_us
send_sigtrap(regs, 0, get_si_code(dr6));

out_irq:
- local_irq_disable();
+ irqentry_irq_disable(regs);
out:
instrumentation_end();
irqentry_exit_to_user_mode(regs);
@@ -1064,7 +1052,7 @@ static void math_error(struct pt_regs *r
char *str = (trapnr == X86_TRAP_MF) ? "fpu exception" :
"simd exception";

- cond_local_irq_enable(regs);
+ irqentry_irq_enable(regs);

if (!user_mode(regs)) {
if (fixup_exception(regs, trapnr, 0, 0))
@@ -1099,7 +1087,7 @@ static void math_error(struct pt_regs *r
force_sig_fault(SIGFPE, si_code,
(void __user *)uprobe_get_trap_addr(regs));
exit:
- cond_local_irq_disable(regs);
+ irqentry_irq_disable(regs);
}

DEFINE_IDTENTRY(exc_coprocessor_error)
@@ -1160,7 +1148,7 @@ static bool handle_xfd_event(struct pt_r
if (WARN_ON(!user_mode(regs)))
return false;

- local_irq_enable();
+ irqentry_irq_enable(regs);

err = xfd_enable_feature(xfd_err);

@@ -1173,7 +1161,7 @@ static bool handle_xfd_event(struct pt_r
break;
}

- local_irq_disable();
+ irqentry_irq_disable(regs);
return true;
}

@@ -1188,12 +1176,12 @@ DEFINE_IDTENTRY(exc_device_not_available
if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
struct math_emu_info info = { };

- cond_local_irq_enable(regs);
+ irqentry_irq_enable(regs);

info.regs = regs;
math_emulate(&info);

- cond_local_irq_disable(regs);
+ irqentry_irq_disable(regs);
return;
}
#endif
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1209,6 +1209,12 @@ do_kern_addr_fault(struct pt_regs *regs,
NOKPROBE_SYMBOL(do_kern_addr_fault);

/*
+ * EFLAGS[3] is unused and ABI defined to be 0, use it to store IRQ state,
+ * because do_user_addr_fault() is too convoluted to track things.
+ */
+#define X86_EFLAGS_MISC (1UL << 3)
+
+/*
* Handle faults in the user portion of the address space. Nothing in here
* should check X86_PF_USER without a specific justification: for almost
* all purposes, we should treat a normal kernel access to user memory
@@ -1290,13 +1296,11 @@ void do_user_addr_fault(struct pt_regs *
* User-mode registers count as a user access even for any
* potential system fault or CPU buglet:
*/
- if (user_mode(regs)) {
- local_irq_enable();
+ if (user_mode(regs))
flags |= FAULT_FLAG_USER;
- } else {
- if (regs->flags & X86_EFLAGS_IF)
- local_irq_enable();
- }
+
+ irqentry_irq_enable(regs);
+ regs->flags |= X86_EFLAGS_MISC;

perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

@@ -1483,14 +1487,10 @@ handle_page_fault(struct pt_regs *regs,
do_kern_addr_fault(regs, error_code, address);
} else {
do_user_addr_fault(regs, error_code, address);
- /*
- * User address page fault handling might have reenabled
- * interrupts. Fixing up all potential exit points of
- * do_user_addr_fault() and its leaf functions is just not
- * doable w/o creating an unholy mess or turning the code
- * upside down.
- */
- local_irq_disable();
+ if (regs->flags & X86_EFLAGS_MISC) {
+ regs->flags &= ~X86_EFLAGS_MISC;
+ irqentry_irq_disable(regs);
+ }
}
}

--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -7,6 +7,7 @@
#include <linux/syscalls.h>
#include <linux/seccomp.h>
#include <linux/sched.h>
+#include <asm/ptrace.h>

#include <asm/entry-common.h>

@@ -218,6 +219,24 @@ static inline void local_irq_disable_exi
}
#endif

+static inline void irqentry_irq_enable(struct pt_regs *regs)
+{
+ if (!regs_irqs_disabled(regs)) {
+ local_irq_enable();
+ if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
+ umcg_sys_enter(regs, 0);
+ }
+}
+
+static inline void irqentry_irq_disable(struct pt_regs *regs)
+{
+ if (!regs_irqs_disabled(regs)) {
+ if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
+ umcg_sys_exit(regs);
+ local_irq_disable();
+ }
+}
+
/**
* arch_exit_to_user_mode_work - Architecture specific TIF work for exit
* to user mode.
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