[Devel] [PATCH RHEL10 COMMIT] ve/printk: Add deferred version for scheduler-safe logging

[Konstantin Khorenko]

The commit is pushed to "branch-rh10-6.12.0-55.13.1.2.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-55.13.1.2.21.vz10
------>
commit 6f133010a39b5a3bd5e0b7103bb9a34515c2ff8f
Author: Aleksei Oladko <aleksey.oladko at virtuozzo.com>
Date:   Tue Nov 25 16:40:31 2025 +0000

    ve/printk: Add deferred version for scheduler-safe logging
    
    Introduce ve_printk_deferred() that uses LOGLEVEL_SCHED to defer console
    output, preventing deadlocks when called from scheduler contexts where
    up() cannot be invoked.
    
    The function mirrors ve_printk() logic but uses vprintk_emit() with
    LOGLEVEL_SCHED instead of vprintk(), ensuring console output is offloaded
    to kthreads rather than printed directly.
    
    https://virtuozzo.atlassian.net/browse/VSTOR-119876
    
    Signed-off-by: Aleksei Oladko <aleksey.oladko at virtuozzo.com>
    Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    
    Feature: printk: per-CT virtualization
---
 include/linux/printk.h |  8 ++++++++
 kernel/printk/printk.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index d74ad6f389870..8bbf98d43b939 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -169,6 +169,9 @@ int _printk(const char *fmt, ...);
 asmlinkage __printf(2, 3) __cold
 int ve_printk(int dst, const char *fmt, ...);
 
+asmlinkage __printf(2, 3) __cold
+int ve_printk_deferred(int dst, const char *fmt, ...);
+
 struct ve_struct;
 int ve_log_init(struct ve_struct *ve);
 void ve_log_destroy(struct ve_struct *ve);
@@ -237,6 +240,11 @@ int ve_printk(int dst, const char *s, ...)
 {
 	return 0;
 }
+static inline __printf(2, 3) __cold
+int ve_printk_deferred(int dst, const char *s, ...)
+{
+	return 0;
+}
 static inline
 int ve_log_init(struct ve_struct *ve)
 {
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f8e32b5fad0d7..68d82ff5a1fa3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2558,6 +2558,34 @@ asmlinkage int ve_printk(int dst, const char *fmt, ...)
 }
 EXPORT_SYMBOL(ve_printk);
 
+static asmlinkage int ve_vprintk_deferred(int dst, const char *fmt, va_list args)
+{
+	va_list args2;
+	int r = 0;
+
+	va_copy(args2, args);
+	if (ve_is_super(get_exec_env()) || (dst & VE0_LOG))
+		r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
+	if (!ve_is_super(get_exec_env()) && (dst & VE_LOG))
+		r = ve_vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args2);
+	va_end(args2);
+
+	return r;
+}
+
+asmlinkage int ve_printk_deferred(int dst, const char *fmt, ...)
+{
+	va_list args;
+	int r;
+
+	va_start(args, fmt);
+	r = ve_vprintk_deferred(dst, fmt, args);
+	va_end(args);
+
+	return r;
+}
+EXPORT_SYMBOL(ve_printk_deferred);
+
 static asmlinkage int ve_log_vprintk(struct ve_struct *ve, const char *fmt, va_list args)
 {
 	int r = 0;